The present invention relates to a closure material for sealing glass containers.
Bottles or like glass containers filled with powdery, liquid or fluid food are sealed usually by crimping a metal cap around the mouth of the container, or firmly screwing a threaded cap on the mouth, or fixedly fitting a crown around the mouth. However, such a cap or crown has the problem of being relatively difficult to open and costly.
It is also practice to coat the lip of glass bottles with a thermoplastic resin and intimately affix a closure material to the resin coating by heat sealing. This method nevertheless involves the likelihood that the resin of the closure will partially remain on the lip when the container is opened to impair the appearance of the container, while the coating of the lip with the resin in advance requires an additional step to render the container inefficient and costly to manufacture.
In the case of glass containers containing powder food, it is also practice to affix a closure to the mouth of the glass container with an adhesive, but this method has the problem that the closure is difficult to remove when opening the container and permits the adhesive to remain on the container mouth portion.
Further in recent years, a method has been developed wherein a closure coated with a thermally adhesive resin is adhered to the mouth of glass containers by heat sealing without coating the lip of the container with resin. The thermally adhesive resin used for this method is, for example, a resin obtained by neutralizing with a metal ion a copolymer of ethylene and .alpha., .beta.-ethylenically unsaturated carboxylic acid (see Published Unexamined Japanese Patent Application Laid-Open No. SHO 06-27533), or a resin prepared by graft-polymerizing an unsaturated carboxylic acid with a hydrolyzed product of ethylene-vinyl acetate copolymer (see Published Unexamined Japanese Patent Application Laid-Open No. SHO 58-20604 which corresponds to U.S. Pat. No. 4,442,129) the former resin is used for this method, the lip of the glass container needs to be coated with a metallic oxide in advance. When the latter resin is used, it is required to preheat the glass container before it is filled with food and to heat-seal the closure usually at a high temperature of 210.degree. to 280.degree. C., hence a cumbersome sealing procedure. Moreover, the conventional method which requires a high temperature for heat sealing is not usable for dessert and chilled food which must be maintained at a temperature lower than room temperature and has the problem of requiring a high equipment cost for the filling-sealing apparatus and a high operating cost because of the high heat-sealing temperature.